Thermal head

ABSTRACT

In the present invention, with an aim of providing a thermal head capable of being driven with reduced power requirements by preventing the thermal efficiency from being deteriorated due to thermal diffusion, the first embodiment is characterized in that on top of a substrate, there is formed a heat insulating layer made up of an electrically-conductive multiple low oxide ceramic layer of low thermal conductivity made of a chemical compound of Si, plural transition metals and oxygen; and that there are stacked and formed, on top of the heat insulating layer, a first insulating layer made up of an insulated multiple high nitride ceramic layer of low thermal conductivity made of a chemical compound of at least Si, plural transition metals and nitrogen, and on top of the first insulating layer, a second insulating layer made up of an SiO 2  layer of high insulation characteristics or an Al 2 O 3  layer. The second embodiment is characterized in that the substrate is made of glass whose composition is phase-separated by means of heat treatment; that there is formed a projecting section protruding at a predetermine height at a position where the heating element is formed; on the projecting section, there is partially formed a first heat insulating layer made up of a porous glass layer obtained by selectively eluting one composition of the glass whose phase has been separated; and that further on top of the first heat insulating layer, there is stacked and formed a second heat insulating layer made of ceramic of low thermal conductivity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a thermal head for use with athermal printer, and more particularly to a power-thrifty thermal headhaving high-speed thermal responsivity.

[0003] 2. Description of the Related Art

[0004] In recent years, the thermal head has been heavily used for arecording device for various information apparatuses. Thus, in order tospeed up, reduce the prices of, reduce power requirements of, andminiaturize these information apparatuses, a power-thrifty thermal headhaving high-speed thermal responsivity has also been requested.

[0005] In order to realize such a thermal head having high-speed thermalresponsivity, for heat insulating layer material, material with thermalconductivity decreased can be used, and the thickness of the heatinsulating layer can be made thinner to reduce the thermal capacity.However, it is technically difficult to decrease the thermalconductivity of a glaze heat insulating layer material which hasconventionally been used, and when the glaze heat insulating layer isused, the thermal capacity of the glaze heat insulating layer has beendecreased by simply forming to make the thickness thereof thinner, andthe reserve heat has been reduced.

[0006] For the reason, high-speed printing has generally been performedwith desired thermal responsivity while sacrificing the electric powersaving.

[0007] With reference to FIG. 5, the description will be made of such aconventional thermal head. On the top surface of a substrate 1 with heatdissipating property, made of alumina or the like, there is formed aglaze heat insulating layer 2 made of glass, having thermal conductivityof nearly 1.1 W/m.k.

[0008] This glaze heat insulating layer 2 is formed so as to be as thickas, for example, 200 μm in thickness, and is formed with a projectingsection 2 a in which a portion formed with a heating element 3 a to bedescribed later protrudes at a predetermined height.

[0009] Also, on the top surface of the glaze heat insulating layer 2,there is formed a heating resistor 3 made of Ta—SiO₂, TiO₂ and the like,and on the top surface of this heating resistor 3, there are formed acommon power feeding member 4 and an individual power feeding member 5.In a portion sandwiched between this common power feeding member 4 andthe individual power feeding member 5, there is formed a heating element3 a.

[0010] On top of these components, a protective layer 6 made of ceramicsuch as Thialone is covered so as to prevent the heating element 3 a orthe common power feeding member 4, the individual power feeding member 5and the like from being oxidized or worn.

[0011] In the conventional thermal head having such a structure, theindividual power feeding member 5 is pulse-energized on the basis ofprinting information, whereby the heating element 3 a is adapted to beable to selectively generate heat.

[0012] However, since such a conventional thermal head as describedabove has been only the glaze heat insulating layer 2 as extremely thickas, for example, 200 μm in thickness formed on top of the substrate 1with heat dissipating property, it has the same thermal conductivity.

[0013] For the reason, the exothermic temperature due to single pulseenergization while the glaze heat insulating layer 2 has been coolbecomes a low exothermic temperature without regard to the thickness ofthe glaze heat insulating layer 2. Accordingly, since great appliedenergy is requested during energization, there has been the problem thatat the head of line at the commencement of printing, no power savingeffect has been obtained, and a peak current of the battery cannot bereduced.

[0014] Also, when pulse energization is continuously performed, reserveheat in a portion in which the heating element 3 a with great printingduty has been formed in the glaze heat insulating layer 2 remarkablyincreases because of inferior heat dissipating property of the glazeheat insulating layer 2.

[0015] For the reason, the exothermic temperature of the heating element3 becomes excessively high, and exceeds a control range of control ofenergized heat, which might possibly cause deteriorated printing qualitydue to blotting, tailing or the like on an image printed on a recordingsheet.

[0016] However, in recent years, there has been disclosed a heatinsulating layer material, of which power saving does not have to besacrificed, and which is excellent in thermal responsivity. Such heatinsulating layer material is obtained by forming low oxide ceramic withlow thermal conductivity by means of oxygen reactive sputteringdeposition.

[0017] With reference to FIG. 6, the description will be made of aconventional thermal head using such a heat insulating layer material asdescribed above. On the surface of a substrate 11 excellent in heatdissipating property, made of silicon or the like, there is formed aprojecting section 11 a having a predetermined height by means of thephotolithography technique, and on top of this projecting section 11 a,a heat insulating layer 12 is stacked and formed.

[0018] The heat insulating layer 12 is made up of Si, plural transitionmetals and oxygen, has low thermal conductivity and electricalconductivity, having thermal conductivity of nearly 0.8 W/m.k andelectrical resistivity of nearly 100 Ω-cm, and is formed to have athickness of 10 to 30 μm on the substrate 11 by means of the oxygenreactive sputtering deposition.

[0019] Also, on the top surface of the heat insulating layer 12, inorder to impart insulation characteristics and resistance to etching tothe surface, an insulating layer 13 made of ceramic with insulationcharacteristics such as SiO₂ and Al₂O₃ is stacked and formed in a singlelayer at a thickness of nearly 2 μm by means of the sputteringdeposition or the like.

[0020] Also, on the top surface of the insulating layer 13 as a singlelayer, a heating resistor 14 made of Ta—SiO₂, Ti—SiO₂ and the like isstacked by means of the sputtering deposition or the like, and a patternof the heating resistor 14 is formed by means of the photolithographytechnique.

[0021] On the top surface of the heating resistor 14, a common powerfeeding member 15 and an individual power feeding member 16 which ismade of Al, Cu and the like are formed, and a portion sandwiched betweenthis common power feeding member 15 and the individual power feedingmember 16 is formed with a heating element 14 a.

[0022] On top of these components, a protective layer 17 made of ceramicsuch as Thialone is covered so as to prevent the heating element 14 a,the common power feeding member 15, the individual power feeding member16 and the like from being oxidized or worn.

[0023] Since there is formed a heat insulating layer 12 of low thermalconductivity on top of a silicon substrate 11 of high thermalconductivity, the conventional thermal head having such a structure iscapable of obtaining high exothermic temperature at a heating element 14a due to single pulse energization to be singly driven, and decreasingprinting blurring at the head of lines where the temperature of thesubstrate 11 is low at the commencement of printing.

[0024] Also, peak current of the power supply is decreased and it ispossible to reduce power requirements and to miniaturize the powersupply. Also, on account of a combination of the substrate 11 excellentin heat dissipating property with the heat insulating layer 12 with lowthermal capacity excellent in heat insulating properties due tocontinuous pulse energization to be continuously driven, even incontinuous energization, it is possible to gently raise the temperatureat the substrate 11 due to reserve heat and to provide a thermal headexcellent in high-speed printing with power requirements reduced.

[0025] However, such a conventional thermal head using the heatinsulating layer material as described above has had the problem thateven if, on top of the heat insulating layer 12 of electricalconductivity, the heating resistor 14 and the power feeding members 15and 16 are stacked through the insulating layer 13 made of ceramic ofinsulation characteristics as a single layer, current leakage occursbetween each pattern through the heat insulating layer 12 because of pinhole defect peculiar to vapor deposition.

[0026] When such current leakage occurs, the thermal head may not beable to be properly operated.

[0027] Also, even if the thickness of the insulating layer 13 wasincreased to nearly 2 μm, the leakage could not be completelyeliminated. When the film thickness of the insulating layer 13 isfurther increased, since the insulating layer 13 had higher thermalconductivity than the heat insulating layer 12, there was the problemthat thermal diffusion in the insulating layer 13 increased todeteriorate the thermal efficiency.

[0028] Further, in order to increase the thickness of the insulatinglayer 13 formed as a single-ply film to nearly 2 μm through the use ofmaterial such as SiO₂ and Al₂O₃ which is slow in sputtering speed, therewas the problem that the production time becomes long and theproductivity is deteriorated.

[0029] Also, as means for eliminating the pin hole defect peculiar tothe vapor deposition, it is effective to divide and form as a film andto interpose a mechanical washing process therebetween, but there wasthe problem that the addition of such a washing process increased theman-hour to deteriorate the productivity.

SUMMARY OF THE INVENTION

[0030] The present invention has been achieved in views of such problemsas described above, and is aimed to provide a thermal head suitable forhigh-speed printing with reduced power requirements, which eliminatesleakage and prevents the thermal efficiency from being deteriorated dueto thermal diffusion by forming plural layers of insulating layers ofdifferent materials on top of the heat insulating layer.

[0031] As first means for achieving the above-described object, there isprovided a thermal head according to the present invention, including: aheat insulating layer formed on a top surface of a substrate; pluralheating elements made up of plural heating resistors and power feedingmembers on the top surface of the heat insulating layer; and aprotective layer for covering at least surfaces of the heating resistorand the power feeding member, wherein the structure is arranged suchthat the heat insulating layer is made up of an electrically-conductivemultiple low oxide ceramic layer of low thermal conductivity made of achemical compound of Si, plural transition metals and oxygen, and plurallayers of insulating layers with different materials are formed on topof the heat insulating layer.

[0032] Also, as second means for achieving the above-described object,there is provided a thermal head according to the present invention,wherein the insulating layer is constructed such that a first insulatinglayer made up of an insulated multiple high nitride ceramic layer of lowthermal conductivity which is a chemical compound of at least Si, pluraltransition metals and nitrogen, and on top of the first insulatinglayer, a second insulating layer made up of a SiO₂ layer of highinsulation characteristics or an Al₂O₃ layer are stacked and formed.

[0033] Further, as third means for achieving the above-described object,there is provided a thermal head according to the present invention,including: a heat insulating layer formed on a top surface of asubstrate; plural heating elements formed of plural heating resistorsand power feeding members on the top surface of the heat insulatinglayer; and a protective layer for covering at least surfaces of theheating resistors and the power feeding members, wherein the structureis arranged such that the substrate is made of glass whose compositionis phase-separated by means of heat treatment and caused to protrude ata predetermine height at a position where the heating element is formedto form a projecting section; and the heat insulating layer is made intoa two-layer structure of: a first heat insulating layer made up of aporous glass layer obtained by selectively eluting one composition ofthe glass whose phase is separated, which is partially formed on theprojecting section; and a second heat insulating layer made of ceramicof low thermal conductivity stacked and formed on the first heatinsulating layer.

[0034] Further, as fourth means for achieving the above-describedobject, there is provided a thermal head according to the presentinvention, wherein the structure is arranged such that there is formed amask layer for covering the top surface of the substrate including theprojecting section, wherein the mask layer is formed with an opening forpartially forming the first heat insulating layer at an apex of theprojecting section, and wherein the apex of the projecting section isexposed from this opening.

[0035] Further, as fifth means for achieving the above-described object,there is provided a thermal head according to the present invention,wherein the structure is arranged such that the mask layer is made ofceramic of insulation characteristics having a thickness of 0.01 to 0.1μm, that the first heat insulating layer is partially formed from theopening, and that the second heat insulating layer is formed on the topsurfaces of the first heat insulating layer and the mask layer.

[0036] Further, as sixth means for achieving the above-described object,there is provided a thermal head according to the present invention,wherein the structure is arranged such that the first heat insulatinglayer is made up of a porous silicon layer of high heat insulatingproperties, having thermal conductivity of 0.3 to 0.5 W/m.k.

[0037] Further, as seventh means for achieving the above-describedobject, there is provided a thermal head according to the presentinvention, wherein the structure is arranged such that the second heatinsulating layer is made up of a ceramic layer of low thermalconductivity having thermal conductivity of 0.8 to 1.0 W/m.k, made of achemical compound of Si, plural transition metals and oxygen.

[0038] Further, as eighth means for achieving the above-describedobject, there is provided a thermal head according to the presentinvention, wherein the second heat insulating layer is constructed tobecome flat by eliminating a difference in level corresponding to athickness of the mask layer which is caused by following a shape of theopening.

[0039] Further, as ninth means for achieving the above-described object,there is provided a thermal head according to the present invention,wherein the second heat insulating layer is constructed by stacking at athickness of 10 to 30 μm on top of the first heat insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a cross sectional view showing a principal part of athermal head according to a first embodiment of the present invention;

[0041]FIG. 2 is a graph for explaining heat insulation characteristicsconcerning the thermal head according to the first embodiment of thepresent invention;

[0042]FIG. 3 is a cross sectional view showing a principal part of athermal head according to a second embodiment of the present invention;

[0043]FIG. 4 is a graph for explaining heat insulation characteristicsconcerning the thermal head according to the second embodiment of thepresent invention;

[0044]FIG. 5 is a cross sectional view showing a principal part of afirst example of a conventional thermal head; and

[0045]FIG. 6 is a cross sectional view showing a principal part of asecond example of the conventional thermal head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Hereinafter, with reference to the drawings, the description willbe made of the thermal head according to the present invention.

[0047]FIG. 1 is a cross sectional view showing a principal part of athermal head according to a first embodiment of the present invention,and FIG. 2 is a graph showing comparisons in thermal responsivitybetween the thermal head according to the first embodiment of thepresent invention and the conventional thermal head.

[0048] First, the thermal head according to the first embodiment of thepresent invention is, as shown in FIG. 1, formed with a projectingsection 21 a, whose cross section is shaped substantially like atrapezoid, smoothly protruding at height of 2 to 20 μm on the surface ofa substrate 21 made of silicon or the like, excellent in heatdissipation property.

[0049] On the top surface of the substrate 21, there is formed a black,low-density heat insulating layer 22 having electrical conductivityincluding metal, excellent in mechanical strength and heat insulationproperties.

[0050] This heat insulating layer 22 has as low thermal conductivity asnearly 0.8 W/m.k, is made up of plural transition metals and oxygen, andstacked at a thickness of 10 to 30 μm for forming by means of the oxygenreactive sputtering deposition. Also, the heat insulating layer 22 isformed as a film to have electrical conductivity with electricalresistivity of nearly 100 Ω-cm.

[0051] Further, on the top surface of the heat insulating layer 22,there are formed plural layers of insulating layers 23 with differentmaterials. These insulating layers 23 have, in the lower layer, a firstinsulating layer 23 a, formed, made of low thermally-conductiveinsulating multiple high nitride ceramic with a thickness of 0.2 to 1.0μm with Si of the main material as Si₃N₄ of high insulationcharacteristics.

[0052] Also, in the upper layer, there is stacked a second insulatinglayer 23 b made up of a high insulating SiO₂ layer or an Al₂O₃ layerwith a thickness of nearly 0.3 μm, and plural layers of insulating layer23 is formed. In other words, the insulating layer 23 is obtained bystacking at least the first and second insulating layers 23 a and 23 bfor forming.

[0053] Also, on top of the second insulating layer 23 b, a pattern ofthe heating resistor 25 made of Ta—SiO₂ and the like is stacked at athickness of nearly 0.1 μm.

[0054] On top of the heating resistor 25, there are formed a commonpower feeding member 26 and an individual power feeding member 27 whichare made of Al, Cu and the like, and a heating element 25 a is formed ina portion sandwiched between the common power feeding member 26 and theindividual power feeding member 27.

[0055] Also, in order to prevent at least the heating resistor 25, thecommon power feeding member 26 and the individual power feeding member27 from being oxidized and worn, on top of these components, aprotective layer 28 made of ceramic such as Thialone is stacked andcovered at a thickness of nearly 5 μm.

[0056] The description will be made of a manufacturing method for athermal head having such structure according to the present invention.

[0057] First, on the surface of a substrate 21 with high heatdissipating property, a projecting section 21 a with a height of 2 to 20μm is projectingly formed by means of the photolithography technique.

[0058] Next, on the top surface of the substrate 21, a black heatinsulating layer 22 is formed by means of the oxygen reactive sputteringdeposition. In other words, the heat insulating layer 22 is obtained bysputter-depositing in a black, low-density low oxide ceramic film withthe degree of oxidization controlled and has very low thermalconductivity.

[0059] After the completion of the oxygen reactive sputter deposition ofthe heat insulation layer 22, the oxygen gas is replaced with nitrogengas within the same apparatus while the vacuum within the apparatus ismaintained to perform the nitrogen reactive sputter deposition in a highnitrogen atmosphere for continuously forming a first insulating layer 23a made of insulating multiple high nitride ceramic with low thermalconductivity.

[0060] On top of this first insulating layer 23 a, there is stacked andformed a second insulating layer 23 b made of an SiO₂ layer with highinsulation characteristics or an Al₂O₃ layer by means of the sputterdeposition.

[0061] This second insulating layer 23 b may be formed after theprotrusion defect on the surface is removed by polishing the surface ofthe fist insulating layer 23 a after the formation of the firstinsulating layer 23 a. On the surface of the first insulating layer 23 athus polished, there is formed the second insulating layer 23 a isformed, whereby it is possible to securely eliminate the leakage.

[0062] Next, on top of the second insulating layer 23 b, the heatingresistor 25 is stacked and formed by means of the sputter deposition toform a pattern of the heating resistor 25 by means of thephotolithography technique.

[0063] Next, on top of the heating resistor 25, the power feeding membermaterial is stacked by means of the sputter deposition to form thecommon power feeding member 26 and the individual power feeding member27.

[0064] Next, on top of at least the heating resistor 25, the commonpower feeding member 26 and the individual power feeding member 27, theprotective layer 28 is stacked and covered by means of the sputterdeposition.

[0065] Since the first insulating layer 23 a is formed in continuationof the film formation of the heat insulating layer 22, theabove-described thermal head according to the present invention iscapable of enhancing the reliability on the insulation characteristicseven if, on top of the first insulating layer 23 a, the secondinsulating layer 23 b made of SiO₂ or Al₂O₃ is stacked and formed asthin as a thickness of nearly 0.3 μm.

[0066] Also, a second insulating layer 23 b having thermal conductivityof the material higher than the heat insulating layer 22 is formed andis formed to become thin in film thickness thereof, whereby the thermalhead according to the first embodiment of the present invention iscapable of preventing the thermal efficiency from being deteriorated dueto thermal diffusion.

[0067] For that reason, as shown in the graph of thermal responsivity ofFIG. 2, the thermal head according to the present invention indicated bya solid line is capable of raising the exothermic temperature per unittime higher than the thermal head according to the related art indicatedby a broken line, and is more suitable for high-speed printing withpower requirements reduced.

[0068] As described above, since plural layers of insulating layers withdifferent materials have been formed on top of the heat insulating layermade up of an electrically-conductive multiple low oxide ceramic layerof low thermal conductivity made of a chemical compound of Si, pluraltransition metals and oxygen, the thermal head according to the firstembodiment of the present invention is capable of eliminating the pinhole defect peculiar to vapor deposition, and eliminating currentleakage.

[0069] Also, since the insulating layer has been formed by stacking afirst insulating layer made up of an insulated multiple high nitrideceramic, layer of low thermal conductivity made of a chemical compoundof at least Si, plural transition metals and nitrogen, and on top ofthis first insulating layer, a second insulating layer made up of a SiO₂layer of high insulating properties or an Al₂O₃ layer, the firstembodiment of the present invention is capable of providing the thermalhead excellent in high-speed thermal responsivity with powerrequirements reduced by eliminating the current leakage.

[0070] Next, with reference to the drawing, the description will be madeof the thermal head according to the second embodiment of the presentinvention. FIG. 3 is a cross sectional view showing a principal partaccording to the present invention, and FIG. 4 is a graph for explainingcomparison in thermal efficiency between the thermal head according tothe second embodiment of the present invention and the conventionalthermal head.

[0071] First, the thermal head according to the second embodiment of thepresent invention has, as shown in FIG. 3, used a substrate 31 made ofglass with a thickness of nearly 1 mm. This substrate 31 has higherthermal diffusivity than the glaze heat insulating layer 2 described inthe Related art, and is made of glass whose composition is easy to bephase-separated by heat treatment.

[0072] On the surface of this substrate 31, there is formed a projectingsection 31 a, whose cross section protrudes substantially like atrapezoid, with a height of 2 to 50 μm by etching based on thephotolithography technique or the like. Thus, after the formation of theprojecting section 31 a, the substrate 31 is heat-treated at atemperature of 500 to 600° C. to phase-separate the composition of theglass.

[0073] Further, after the substrate 31 is subjected to a phaseseparation heat treatment, on the surface of the substrate 31, SiO₂ andthe like are stacked at a thickness of 0.01 to 0.1 μm to form a masklayer 32 for covering the top surface of the substrate 31 including theprojecting section 31 a by means of the photolithography technique.

[0074] In this mask layer 32, on the apex 31 b of the projecting section31 a, there is formed a slit-shaped opening 32 a with a width of nearly50 μm, and from this opening 32 a, a portion of the apex 31 b of theprojecting section 31 a is exposed.

[0075] In this respect, the projecting section 31 a and the mask layer32 may be formed before the substrate 31 is subjected to the phaseseparation heat treatment, and thereafter, may be subjected to the phaseseparation heat treatment.

[0076] Further, the substrate 31 formed with the mask layer 32 isimmersed in heated acid liquid such as hydrochloric acid or nitric acidor thermal water, whereby there is formed a first heat insulating layer33 made up of a porous glass layer in which a glass phase such asNa2O-B2O3 has been selectively eluted on the apex 31 b of the projectingsection 31 a exposed from the opening 32 a.

[0077] The first heat insulating layer 33 has remarkably lower thermalconductivity than the glaze heat insulating layer 2 described in theRelated art, and its thermal conductivity is 0.3 to 0.5 W/m.k. In thisrespect, the first heat insulating layer 33 has a depth of nearly 50 μm,and is formed directly below a heating element 36 a to be describedlater.

[0078] Also, on the top surfaces of the first heat insulating layer 33and the mask layer 32 which are exposed from the opening 32 a, there isformed a second heat insulating layer 34 made of ceramic with lowthermal conductivity, and excellent in mechanical strength and heatinsulating properties.

[0079] Thus, this second heat insulating layer 34 has been stacked andformed at a thickness of 10 to 30 μm by means of the sputter deposition,and also serves to reinforce the first heat insulating layer 33, whichis porous and inferior in mechanical strength.

[0080] In this respect, the second heat insulating layer 34 is made upof Si, plural transition metals and oxygen, and has thermal conductivityof 0.8 to 1.0 W/m.k.

[0081] Also, on the surface of the second heat insulating layer 34, inorder to eliminate a difference in level (not shown) corresponding tothe thickness of the mask layer 32 which is caused by following theshape of the opening 32 a of the mask layer 32, the top surface of thesecond heat insulating layer 34 is made flat by polishing.

[0082] Also, on the top surface of the second heat insulating layer 34which has been made flat by polishing, an undercoat layer 35 made ofceramic with insulation characteristics such as SiO₂ having resistanceto etching has been stacked and formed at a thickness of nearly 0.3 μm.

[0083] On top of this undercoat layer 35, a heating resistor 36 made ofTa—SiO₂ has been stacked at a thickness of nearly 0.1 μm by means of thesputter deposition, and a pattern of the heating resistor has beenformed by means of the photolithography technique.

[0084] Further, on top of the heating resistor 36, power feeding membermaterial made of Al, Cu and the like is stacked at a thickness of 1 to 3μm by means of the sputter deposition, and a common power feeding member37 and an individual power feeding member 38 have been formed by meansof the photolithography technique. In this respect, a portion sandwichedbetween the common power feeding member 37 and the individual powerfeeding member 38 is a heating element 36 a.

[0085] This heating element 36 a is formed at such a position as thefirst heat insulating layer 33 comes directly below the heating element36 a.

[0086] Thus, in order to prevent at least the heating resistor 36, thecommon power feeding member 37 and the individual power feeding member38 from being oxidized or worn, on top of these components, a protectivelayer 39 made of ceramic such as Thialone is stacked and covered at athickness of nearly 5 μm by means of the sputter deposition.

[0087] As described above, the thermal head according to the secondembodiment of the present invention has been formed by stacking a firstheat insulating layer 33 made up of a porous SiO₂ layer with high heatinsulating properties having thermal conductivity of 0.3 to 0.5 W/m.k,and on top of this first heat insulating layer 33, a second heatinsulating layer 34 with low thermal conductivity of 0.8 to 1.0 W/m.k.Therefore, since this thermal head has remarkably higher heat insulatingproperties than the conventional glaze heat insulating layer 2, a heatflow generated in the heating element 36 a receives high heat resistanceof the first and second heat insulating layers 33 and 34 to decrease anamount of heat into the substrate 31.

[0088] Thus, since the reaction increases the amount of heat on theprint medium (not shown) side with which the heating element 36 a isbrought into press contact, the thermal head can be used as apower-thrifty thermal head with high thermal efficiency which is optimumparticularly for use with a portable printer and the like to be batterydriven.

[0089] In such a thermal head according to the second embodiment of thepresent invention, although the substrate 31 is a glass substrate havinglow thermal diffusivity, due to an operation of the first and secondheat insulating layers 33 and 34 with high heat insulating propertieswhich have been formed in two layers, a temperature rise in the thermalhead according to the second embodiment of the present inventionindicated by a solid line becomes gentler than the thermal headaccording to the related art indicated by a broken line as shown in FIG.4 even if printing is made continuously, and a time period untilblotting or tailing failure due to excessive printing density occurs,can be extended. For that reason, a number of times of stoppage forcooling during printing also becomes less, and it is possible to reducea decline in the throughput which is effective printing speed.

[0090] As described above, the thermal head according to the secondembodiment of the present invention has been made into a two-layerstructure of: a first heat insulating layer made up of a porous glasslayer obtained by selectively eluting one composition of the glass whosephase has been separated, which has been partially formed on theprojecting section; and a second heat insulating layer made of ceramicwith low thermal conductivity stacked and formed on this first heatinsulating layer. Therefore, it is possible to provide a power-thriftythermal head with high thermal efficiency, which is excellent inmechanical strength and optimum for use with a portable printer or thelike.

[0091] Further, there is formed a mask layer for covering the topsurface of the substrate including the projecting section, in this masklayer, there is formed an opening for partially forming the first heatinsulating layer at the apex of the projecting section, and the apex ofthe projecting section is exposed from this opening. Therefore, thesubstrate formed with the mask layer is immersed in acid liquid orthermal water, whereby it is possible to easily form a first heatinsulating layer made up of a porous glass layer on the apex of theprojecting section exposed from the opening, and to provide apower-thrifty thermal head easy to be manufactured.

[0092] Further, since the first heat insulating layer is made up of aporous silicon layer with high heat insulating properties, havingthermal conductivity of 0.3 to 0.5 W/m.k, a thermal head with highthermal efficiency can be provided.

[0093] Further, since the mask layer is made of ceramic with insulationcharacteristics having a thickness of 0.01 to 0.1 μm, the first heatinsulating layer is partially formed from the opening, and on the topsurfaces of this first heat insulating layer and the mask layer, thereis formed the second heat insulating layer, it is easy to manufacturethe second heat insulating layer which also serves as a reinforcementlayer for the first heat insulating layer.

[0094] Further, since the second heat insulating layer is made up of aceramic layer with low thermal conductivity having thermal conductivityof 0.8 to 1.0 W/m.k, made of a chemical compound of Si, pluraltransition metals and oxygen, this second heat insulating layer and thefirst heat insulating layer are made into a two-layer structure, wherebyit is possible to provide a thermal head with high thermal efficiency,excellent in mechanical strength.

[0095] Further, since the second heat insulating layer has been madeflat by eliminating a difference in level corresponding to the thicknessof the mask layer which is caused by following the shape of the opening,it is possible to securely bring the heating element portion on theopening into tight contact with the recording medium or the like, and toprovide a thermal head capable of printing in high quality.

[0096] Further, since the second heat insulating layer has been formedby stacking at thickness of 10 to 30 μm on top of the first heatinsulating layer, it is possible to increase the mechanical strength ofthe second heat insulating layer, to securely reinforce the first heatinsulating layer, which is inferior in mechanical strength, and toimprove the long lasting characteristics and the like.

What is claimed is:
 1. A thermal head, comprising: a heat insulatinglayer formed on a top surface of a substrate; plural heating elementsmade up of plural heating resistors and power feeding members on the topsurface of the heat insulating layer; and a protective layer forcovering at least the surfaces of the heating resistors and the powerfeeding members, wherein the heat insulating layer is made up of anelectrically-conductive multiple low oxide ceramic layer with lowthermal conductivity made of a chemical compound of Si, pluraltransition metals and oxygen, and wherein plural layers of insulatinglayers of different materials are formed on top of the heat insulatinglayer.
 2. The thermal head according to claim 1, wherein the insulatinglayer is formed by stacking a first insulating layer made up of aninsulated multiple high nitride ceramic layer of low thermalconductivity, which is a chemical compound of at least Si, pluraltransition metals and nitrogen, and on top of the first insulatinglayer, a second insulating layer made up of an SiO₂ layer of highinsulation characteristics or an Al₂O₃ layer.
 3. A thermal head,comprising: a heat insulating layer formed on a top surface of asubstrate; plural heating elements formed of plural heating resistorsand power feeding members on the top surface of the heat insulatinglayer; and a protective layer for covering at least surfaces of theheating resistors and the power feeding members, wherein the substrateis made of glass whose composition is phase-separated by means of heattreatment and caused to protrude at a predetermine height at a positionwhere the heating element is formed to form a projecting section, andwherein the heat insulating layer is made into a two-layer structure of:a first heat insulating layer made up of a porous glass layer obtainedby selectively eluting one composition of the glass whose phase isseparated, which is partially formed on the projecting section; and asecond heat insulating layer made of ceramic of low thermal conductivitystacked and formed on top of the first heat insulating layer.
 4. Thethermal head according to claim 3, wherein there is formed a mask layerfor covering the top surface of the substrate including the projectingsection, wherein the mask layer is formed with an opening for partiallyforming the first heat insulating layer at an apex of the projectingsection, and wherein the apex of the projecting section is exposed fromthe opening.
 5. The thermal head according to claim 4, wherein the masklayer is made of ceramic of insulation characteristics having athickness of 0.01 to 0.1 μm, wherein the first heat insulating layer ispartially formed from the opening, and wherein the second heatinsulating layer is formed on the top surfaces of the first heatinsulating layer and the mask layer.
 6. The thermal head according toclaim 3, wherein the first heat insulating layer is made up of a poroussilicon layer of high heat insulating properties, having thermalconductivity of 0.3 to 0.5 W/m.k.
 7. The thermal head according to claim6, wherein the second heat insulating layer is made up of a ceramiclayer of low thermal conductivity having thermal conductivity of 0.8 to1.0 W/m.k, made of a chemical compound of Si, plural transition metalsand oxygen.
 8. The thermal head according to claim 5, wherein the secondheat insulating layer is made flat by eliminating a difference in levelcorresponding to a thickness of the mask layer which is caused byfollowing a shape of the opening.
 9. The thermal head according to claim8, wherein the second heat insulating layer is formed by stacking at athickness of 10 to 30 μm on top of the first heat insulating layer.